Before starting on how you can be supportive of someone who has OCD, I would like to emphasize the importance of taking care of your own well- being too. Investing time and energy into helping others will only be helpful or productive if you leave enough for yourself.
Sounds simple enough, but looking at my own partner I know how hard this can be at times.

Helping out with rituals is not the best way to show you love this person, like not participating in them is not showing a lack of love either..

Personal experience has shown me how slowly but steadily a loved- 1 will join in with the rituals. The intentions never start off as being selfish. When you see someone struggle with things that for you are so simple, you probably think, "Where is the harm in doing just this 1 little thing....." But before you know it, you might find yourself 3 years later realizing you have taken on much more than you can or even want to take on and you won't be able to continue doing this (More...).
People that become the "care- taker" will at times put their own feelings and needs aside. Please don't....
You equally deserve to be happy. For your loved 1 to have a rough time, doesn't mean you aren't allowed to make the best of life for yourself. Showing them what is out there is more helpful and motivating than closing yourself in with them.
Having said this, showing understanding and the willingness to listen even when it's tough is something you should always try, to give that much needed support.

By the way, you may have noticed I said that helping with rituals "never" starts off as being something selfish. The reason I say this is because I do feel that after a while, it's almost easier to give in.
When you say no, chances are a fight will start or you will be late at whatever you are going to and so on... There is only so much a person can take, even the strong 1, you may not have OCD you are also only human.
There is no shame in being fed up at times and giving in to a ritual rather than having to watch the struggle of your loved- 1 when they try to confront their OCD.
But if you want to construct rather than stagnate you might have to push yourself and the person some to avoid any rigidity and rules to form too much, just do so in a caring and educated way.

Avoid becoming part of their system (More...), you most likely won't be able to keep up with the person relying more and more on you and so you better decide as early as possible what and what not you will agree on doing (Consult a psychologist for proper guidelines on how to do this.).
A person that has OCD can be really rigid in her/his thinking, their way is the right way when they are anxious, is part of the condition but can be pretty frustrating for those around.
Just try and remain calm when dealing with a person that is going through this and is being confronted with having to make changes in a pattern that is their only way of feeling some relief from their anxiety.
But it's important to show consistency rather than to adapt according to circumstances too much.

Accepting to do something until you are fed up with it, is harder to take than never accepting to do it at all, for the person who has OCD.

But this is easier said than done, so inform yourself as much as you can and preferably find a professional to help you with any questions you may have.
In all try and be as supportive as you can be, be consistent and remain patient and positive at all times.
Just don't forget yourself either.